1. Field of the Invention
This invention relates to a picture-quality improving circuit usable in various systems such as television systems or video systems.
2. Description of the Prior Art
There is a recognized need to sharpen the images of television signals, particularly at transition edges where the picture changes brightness or color, or both brightness and color. In television signal formats such as NTSC and PAL, the color information is encoded on a subcarrier which is interleaved with the baseband luminance information. An inherent drawback of these formats is the limited bandwidth for the color information component, called "chroma" or "chrominance".
There are many chances in a color television system for the chroma information to become degraded, particularly at transitions from one hue to another.
U.S. Pat. No. 4,504,853 discloses a color television processing system which enhances aperiodic transitions occurring in a quadrature modulated subcarrier by deriving a control signal from simultaneously occurring aperiodic transitions in the luminance, without demodulation of the chroma subcarrier. The system of U.S. Pat. No. 4,504,853 includes an input, a delay match for delaying a modulated subcarrier signal by a predetermined amount, a one-half period delay for delaying the modulated subcarrier signal by half the period thereof, and a first adder for combining in equal amounts the undelayed modulated subcarrier with the signal delayed by the one-half period delay to provide a transition envelope. A control generator receives the luminance signal and derives a control signal from transitions occurring in the luminance. A multiplier multiplies the transition envelope signal by the control signal to provide an enhancement product. A second adder combines the delay matched modulated subcarrier in phase with the enhancement product to put out the enhanced modulated subcarrier characterized by shortened transitions in alignment with simultaneous transitions in the baseband.
Since the system of U.S. Pat. No. 4,504,853 uses a correlation between the color signal and the luminance signal in improving the transition characteristics of the color signal, the intended effect is realized when the color signal actually correlates to the luminance signal. In the system of U.S. Pat. No. 4,504,853, when the luminance signal and the color signal are uncorrelated and the color signal changes at a point near a change of the luminance signal, the edge information of the color signal is easily modulated with the edge information of the luminance signal so that the color signal tends to be contaminated by false edge information.